Custom Formatters Not Working

It seams neither my custom Json nor Xml formatters are being executed. I found a previous discussion over this subject which seamed to indicate that it was a bug in Preview 5. Has this been fixed yet? If so, what branch is the fix on? Or, is there a workaround
to get this working? This is a pretty big deal. Basically, nothing works because of this.

Yes, this is a known issue and it is fixed. You can do a local build of the default branch to get the fix, or you can work around the issue by deriving your custom formatter from the default json or xml formatters.